BALB/c x C57B1/6N F sub 1 hybrid mice were immunized with the pneumococcal polysaccharide SSS-III. Some were given simultaneously different numbers of parental lymphoid cells from spleen, lymph node, blood or thymus. Four to seven days later, the numbers of cells forming antibody to SSS-III in spleens of recipients was determined using localized hemolysis in gel of SSS-III coated sheep erythrocytes. Parental cells produced an enhancement (allogeneic effect) of this antibody response. The capacity of cells from different sources or after various treatments such as lysis with anti Thy-1,2 in vitro and administration in vivo of anti thymocyte serum were compared quantitatively.